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Hydrograph separation techniques were used to determine contributions of old and new water during storm events at four sites within the urban Anacostia River watershed. Multiple storm hydrographs were successfully separated with electrical conductivity as a tracer. Total runoff correlated to rainfall, but most runoff ratios were significantly less than the percentage of impervious surfaces. Old water was a significant component of runoff at each site. Peak contributions of old water occurred earlier new water peaks, which suggests rapid transmission of groundwater to streams. New water runoff was the dominant contribution for storm events greater than 2-3 cm. Watershed topography influenced patterns of urbanization and runoff pathways. Riparian buffers along Piedmont streams appeared to be sites of infiltration of overland flow. These results indicate that electrical conductivity is an effective tracer for the evaluation of streamflow sources within large urban watersheds.